1. Field of the Invention
The present invention relates to a method of and apparatus for controlling the quantity of fuel supplied to an internal combustion engine, and particularly, to a technique of properly adjusting a fuel quantity correction level according to a change in fuel characteristics.
2. Description of the Related Art
Internal combustion engines have an electronic fuel injector. When the temperature of the engine is low, fuel from the injector is poorly atomized and adheres around a suction valve, to reduce the fuel drawn into a cylinder and thereby lean an air-fuel ratio. To prevent the adhesion of fuel and the leaning of air-fuel ratio, the fuel supply to the engine must be incrementally corrected according to the temperature of cooling water, which corresponds to the temperature of the engine. (Refer to, for example, Japanese Unexamined Utility Model Publication No. 62-162364.)
An adhesion ratio, i.e., the ratio of the fuel adhered around the suction valve to the injected fuel and an evaporation ratio, i.e., the ratio of the fuel evaporating from the adhered fuel and being drawn into the cylinder to the adhered fuel fluctuate even at the same temperature depending on the characteristics (in particular, the evaporative characteristics) of the fuel.
The leaning of air-fuel ratio may be prevented by incrementally correcting the fuel supply. The level of the incremental correction according to the water temperature is usually determined on fuel having heaviest evaporative characteristics, i.e., fuel that hardly evaporates. Accordingly, the incremental correction usually involves a large margin.
The incremental correction according to the water temperature, therefore, will be excessive for fuel having lighter evaporative characteristics, to make the air-fuel ratio rich to increase fuel consumption and deteriorate exhaust quality.
To solve this problem, the applicant of the present invention has disclosed in Japanese Unexamined Patent Publication No. 4-5846 an electronically controlled fuel supply apparatus that adjusts the level of incremental correction carried out on the supply of fuel according to the temperature of cooling water to a minimum to keep the surge torque of an engine within an allowable range.
Even if the temperature-based incremental correction is adjusted according to the characteristics of fuel, conventional incremental and decremental corrections carried out on the supply of fuel during acceleration and deceleration are based on the fuel having the heaviest evaporative characteristics. Accordingly, a change in the fuel characteristics may cause an excess or a shortage in the acceleration-or deceleration-based correction, to deteriorate operability during the acceleration or deceleration.
It is required, therefore, to adjust the acceleration-or deceleration-based correction according to the fuel characteristics, too. It is difficult, however, to speedily detect surge torque during the acceleration and deceleration. Accordingly, it is difficult to adjust the acceleration-or deceleration-based correction in the same manner as for the correction based on the temperature of cooling water.